The present invention relates to enhancing the consistency of responses of electromechanical actuators, and more particularly actuators for braking vehicles, for example, as found in electrically actuated aircraft braking systems.
An illustrative electrically actuated aircraft braking system is shown in U.S. Pat. No. 4,865,162 which teaches a plurality of annularly disposed electrically energizable torque motor and roller screw drive mechanisms for selectively engaging a pressure plate of a wheel and brake assembly and compressing a disc brake stack to retard aircraft motion. In such electrically actuated braking systems, there may be several electromechanical actuators associated with a given wheel which are actuated each time the brakes for that wheel are applied.
Electromechanical actuators including brake actuating motors involve low-torque components geared down to provide high-torque output. Friction and stiction can cause hysteresis between forward and reverse position or torque, and can adversely affect repeatability. Stiction describes a stick-slip effect where static friction interferes with movement in such a way that an actuator appears to xe2x80x9cstickxe2x80x9d short of the desired position and requires additional force before it moves, then sticks again. The word is a contraction of xe2x80x9csticky frictionxe2x80x9d and is sometimes spelled xe2x80x9csticktion.xe2x80x9d
Testing of certain braking systems has shown that, for a given force command, the force varies widely, and is not linear with command. There is also a significant hysteresis; that is, when force command is decreased vs. increased, the decrease does not always match the increase. The electromechanical actuator command-force curve also tends to flatten out at high and low command levels.
Dither is a frequently applied solution to problems of this nature. Dithering involves oscillating the command signal around the control point, causing the actuator to attempt to move backward and forward at high frequency, a frequency to which the actuator cannot fully respond. Dither can sometimes be used to improve accuracy in a positional control system, but requires the system (hardware and software) to be capable of responding to a frequency higher than what would otherwise be required. This additional design requirement can increase cost, weight and power consumption. Another drawback to the dither solution is that the motor is commanded to move in one direction and then the other direction, hence there may be hysteresis due to gear backlash.
It would be highly desirable to move an electromechanical actuator more reliably and repeatably to a commanded position, and to remove hysteresis when moving the actuator in the opposite direction.
The present invention provides solutions to the above problems by interrupting the force application electrically to provide a measured duty cycle for application of force. This duty cycle may be calibrated for period of repetition and for off time within the period of repetition. The repeated application and removal of force overcomes friction and sticktion in the actuator and provides highly repeatable performance. An electronic actuator driver circuit provides separate electrical commands for force and enable of actuator. When the actuator is commanded to a position, the force command remains constant, and the disable command is cycled off and on to provide the required duty cycle. This enable/disable pulse train acts to cause small motor movements which overcome friction and stiction in the reduction gearing.
The invention comprises, in one form thereof, a process of applying a compressive force to a compressible stack of interleaved braking disks to apply a braking force to a rotating wheel and includes providing an operator-initiated digital braking command indicative of a desired braking force and converting that digital command to an analog signal having a magnitude indicative of the desired braking force. The analog signal is periodically interrupted and the interrupted analog signal supplied to a motor. Motor motion is mechanically converted from rotary to linear motion, for example, by a speed reducing, torque increasing gear train and ballscrew assembly, and the linear motion applied to compress the braking disks. Each analog signal interruption is for a time interval less than the time required to disable the motor and remove all braking force from the wheel and preferably, before any free-play motion can occur within the gear train.
An advantage of the present invention is that the force is always applied in the same direction thereby eliminating hysteresis due to gear backlash. Removal of force allows the item acted upon to xe2x80x9cpush backxe2x80x9d on the engaged gear train, maintaining engagement and preventing backlash.
Another advantage is that the delays due to changed command level which may affect dithered signals are absent since command level is unchanged and the force is removed by an independent mechanism.